Fouling refers to the undesirable attachment of organic matter, biomolecules, and microbes on submerged surfaces. Fouling diminishes the performance of devices involving these submerged surfaces and is considered the bottle-neck issue for various applications in the biomedical industry, food processing, and water treatment, especially in reverse osmosis (RO) desalination. Nearly all RO desalination operations use thin-film composite (TFC) membranes due to their excellent salt rejection, low energy cost, and reasonable permeability. However, the surface properties of TFC membranes render them prone to fouling. Fouling reduces permeability and salt rejection ability, and shortens membrane lifetime significantly. Fouling can be reduced or even prevented if chlorine or other oxidizing agents are added to the feed. However, the salt-rejection layer in TFC membranes is polyamide, which is very susceptible to oxidation. Thus, continuous exposure to water containing even a few parts per billion (ppb) chlorine degrades membrane performance significantly. To meet these conflicting requirements, water to be purified is often chlorinated for disinfection purposes, de-chlorinated before entering membrane desalination units, and re-chlorinated after permeating through the membranes. The poor chlorine resistance of TFC membranes leads to significant additional processing steps and, in turn, increases operational costs. Therefore, the prevention of fouling and chlorine oxidation has been identified as the core means to improve the energy usage and reliability of sea water desalination.
Fouling can be reduced by membrane surface modification. Coatings become ineffective once the first layer of foulant forms on the surface and, therefore, extraordinary resistance to fouling or ultra-low fouling is desired. Among the antifouling surface chemistries poly(ethylene glycol) (PEG) brushes are the “gold standard.” However, the poor long-term stability of PEG has been a concern and the unique brush morphology, which is necessary for the ultra-low fouling resistance, is not achievable on all substrates. These largely limit the application of PEG, and reduce its antifouling performance in real-world applications. Zwitterionic chemistry has similar fouling resistance as PEG. Although the betaine structure does not involve unstable bonds (such as ether in PEG), the zwitterionic antifouling polymers are fabricated almost exclusively with acrylate monomers having a relatively labile ester bond that does not stand up to oxidizing agents, such as chlorine.
In addition, current coating techniques (FIG. 6) for zwitterionic chemistry are not substrate-independent and usually involve organic solvents. Coatings made by these methods suffer from surface defects (such as pin-holes) resulting from de-wetting and surface tension effects. In addition, solvent-based techniques can damage delicate substrates, such as RO membranes. A vapor phase deposition method can avoid the potential for this damage by solvents during the surface modification of such membranes.
There exists a need for an efficient, reliable, environmental-friendly, solvent-less method of depositing chlorine-resisting antifouling coatings on reverse osmosis membranes.